This application relates to silicone (polyorganosiloxane) rubber materials and to UV curable compositions for producing same.
One of the most outstanding characteristics of silicone rubber is the retention of virtually all desirable properties over a wide temperature range (-100.degree. to 315.degree. C.), and over a long period at moderate temperature (150.degree. C). For example, silicone rubber has a useful life of up to 20 years at 120.degree. C. and up to 5 years at 150.degree. C.
The following are believed to be typical of heat vulcanized silicone rubber properties:
Mechanical properties: tensile strengths, 200-1500 psi; shore A hardness, 20-80; elongation up to 1000%; and tear strength, up to 150 lb./in.
Electrical properties: dielectric strength, 400-500 volts per mil; dielectric constant, 2.9-3.1 at frequencies of 10.sup.2 -10.sup.10 Hz; power factor 5.times.10.sup.3 at 10.sup.4 Hz to 5.times.10.sup.-2 at 10.sup.9 Hz; and volume resistivity, 10.sup.15 ohm-cm at 37.degree. C. to 10.sup.12 ohm-cm at 260.degree. C.
Miscellaneous: Silicone rubbers typically have good chemical, fuel and oil resistance. Water, in particular, virtually has no effect on them. They have excellent resistance to ozone, sunlight, oxygen and fungus, and display low toxicity and minimum tissue reaction in biological systems. They also have a very high thermal expansion coefficient (2-3.times.10.sup.-4 /.degree.C. for linear thermal expansion coefficient) and a high thermal conductivity (0.7 to 0.2 Cal/Sec/cm.sup.2 /cm/.degree.C.).
The most common method of producing silicone rubbers with useful properties is by compounding gums of high molecular weight polyorganosiloxanes, filler, processing aids and peroxide curing agents and then curing at elevated temperature (150.degree. C. to about 250.degree. C. depending on the peroxide). In order to obtain uniform rubbers, the gum, fillers, additives and peroxide have to be mixed thoroughly in banbury mixers and two-roll mills before they are oven-cured or steam-cured. Because of the extreme difficulty in processing these gums, they have no practical applications as sealants and adhesives. See W. Lynch, "Handbook of Silicone Rubber Fabrication," Van Nostrand Reinhold Co., NY, 1978.
On the other hand, room temperature vulcanizing silicone rubbers (RTV's) employ liquid low molecular weight polyorganosiloxanes, crosslinkers, fillers and catalysts. They are used as one part and two part systems. The one part system normally consists of a hydroxyl terminated polyorganosiloxane, triacetoxy or trimethoxy containing silanes as crosslinkers, organometallic catalysts and fillers. The curing is effected by moisture. The two part system comprises a silicon hydride crosslinker in one part, and a vinyl containing silicone, platinum catalysts, and fillers in the second part. Upon mixing the two parts, the curing takes place. Both types of RTV's generally display the similar properties to the heat cured silicone gum rubbers. Their viscosities range from easily pourable liquids to thixotropic pastes, therefore, they are very useful as adhesives and sealants. However, a complete curing to form rubber usually takes several hours or even days at room temperature, although cure time can be reduced by using elevated temperatures. It is desirable therefore that silicon compositions, which may be easily processed and are capable of very rapid curing to tough silicone rubbers at or near room temperature, be developed.
Ultraviolet (UV) curing silicones containing methacrylate functional groups are known in the art. However, such art does not relate to the formation of silicone rubbers. GB No. 1323869 (1973) taught a composition of (a) an organopolysiloxane containing at least a (meth)acrylate functional group; (2) a photosensitizer; and (3) a solvent for (1) and (2). The composition was intended for plate-making in printing, curing to a hard film, not suitable as a bulk rubber.
U.S. Pat. No. 4,201,808 taught a composition of (a) an organopolysiloxane containing an average of at least one acryloxy and/or methacryloxy group per molecule; (b) a low molecular weight polyacrylyl crosslinking agent; and (c) a photosensitizer. U.S. Pat. No. 4,348,454 also taught a similar composition. These compositions were taught for release coatings and were cured as a thin film by UV.
GB No. 2O39287A taught a composition from the reaction of methacryloxypropyltrimethoxysilane and low molecular weight hydroxyl terminated polyorganosiloxanes. Again, this composition was cured as a thin film for protective coatings on paper, not as a bulk rubber.